SPH 3U
5.1/5.2 What is Energy?
Energy – ability to do ________, scalar
UNIT: Joules (J)
Work – process of transferring _________ from one form to another, scalar
UNIT: Joules (J)
Forms of Energy
Kinetic Energy – energy related to the motion of an object
Ek = _________
m is the mass of the object and v is its speed.
Examples:
Thermal Energy – kinetic energy at the molecular level, as the __________ of an object increases, its
thermal energy increases
Potential Energy – energy that is stored and can be used to do work
-
gravitational potential energy (GPE)  energy that is stored in matter due to its height, where the force
of gravity can act on it
Example: Hydroelectric Power and Energy Conversion
1)
2)
3)
water has gravitational potential energy (GPE)
as it falls, its speed increases, converting GPE into kinetic energy (KE)
falling water causes a turbine to spin, which causes the speed of the water to decrease (KE is
decreasing while work is being done on the turbine)
Kinetic Energy and Work
- a car, whose mass is 3000 kg starts off at 2 m/s and accelerates to 20 m/s
m = 3000 kg
Ek1 = ½mv12
Ek2 = ½mv22
Ek1 = ½(3000)(2)2
Ek2 =
Ek1 = _________
Ek2 = _________
-
the car started off with 20 L of gasoline, however after accelerating may have 19.8 L of gasoline
engine has done work, it has converted the chemical potential energy of the gasoline into kinetic energy
when an object’s speed changes, its kinetic energy changes. To change an object’s kinetic energy, work
must be done on that object. Work is done when a force is applied to an object through a distance
-
under special circumstances (when there is only one force acting on an object), the work done on an
object by a force is equivalent to the change in kinetic energy of the object
W = Ek
For the example above, W = ___________
Other examples include
a) car accelerating, one force
b) car slowing down (braking)
Practice Problems:
1) A car’s speed decreases from 50.4 km/hr to 4 m/s. The mass of the car is 1.5 x 106 grams. Calculate
a) mass in kg b) initial speed in m/s c) initial kinetic energy d) final kinetic energy e) change in Ek
f) work
(Ans: a) 1 500 kg
b) 14 m/s
c) 147 000 J
d) 12 000 J e) – 135 000 J
f) -135 000 J)
2) A car has 14 kilojoules of kinetic energy and travels at 5 m/s. Calculate a) kinetic energy in Joules
b) mass of car
(Ans: a) 14 000 Joules
b) 1120 kg)
3) A car has a mass of 2000 kg and 50 000 J of kinetic energy. Calculate the speed of the car.
(Ans: 7.071 m/s)
What is Work?
-
work is done on an object when a force is applied to an object through a distance
→
→
W = Fappd
-
the formula for work is a special case, when the force is applied in a direction that is parallel to the
direction of motion of the object
a) pushing an object
b) lifting an object
c) lowering an object
-
UNIT: Newton-meter or Joule
d) pushing on a wall
work is positive if the projection of force in the line of displacement is in the same direction as the
displacement
work is negative if the projection of force in the line of displacement is in the opposite direction as the
displacement
-
if the applied force is perpendicular to the direction of motion, then the total work done on the object is
_________
Why?
- sometimes work is done on an object but the object does not gain any energy
ie. Car travelling at a constant speed on the highway  engine does work but car does not gain any speed,
only maintains speed
 Explain
Practice Problems:
1) An object moves 20 m while a force of 5 N is applied to it. Calculate the work done by the force. (Ans:
100 J)
2) An object moves 15 cm while a force of 6 kiloNewtons acts on it. Calculate the work done by the force.
(Ans: 900 J)
3) A car (mass is 1 500 kg) comes to a slow stop. While stopping, the car moves a distance of 20 m and
decreases its speed from 25 m/s to 8 m/s. Calculate
a) initial kinetic energy (Ans: 468 750 J)
b) final kinetic energy (Ans: 48 000 J)
c) change in kinetic energy
(Ans: -420 750 J)
d) work done by the braking force (Ans: -420 750 J) e) size of the braking force
(Ans: 21 037.5
N)
4) A car (mass is 2000 kg) comes to a stop. The size of the braking force is 40 000 N and the initial speed
of the car is 120 km/hr. Calculate the braking distance required to come to a complete stop.
(Ans: 27.78 m)
Gravitational Potential Energy (GPE)
- when an object is lifted, you do work on the object against the force of ___________
- when an object is lifted, it gains ________________ potential energy (GPE (symbol is Eg)
GPE – energy stored in an object due to its height above a reference point
UNIT: Joule
Eg = _________ (h is above a particular reference point)
NB: the value for g is +9.8 N/kg in
energy problems
WHAT HAPPENS TO AN OBJECT’S GRAVITATIONAL POTENTIAL ENERGY WHEN THE OBJECT
FALLS?
- when an object is falling, it loses its gravitational potential energy and is converted into kinetic energy
-
symbol and equation for the change in gravitational potential energy is
EG = m x g x ∆h
Example: A person lifts a book at an angle of 30 degrees (to the horizontal) through a distance of 0.25 m.
Calculate the change in the gravitational potential energy.
Why doesn’t a ball return back to its original height after being dropped?